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# Complex/imaginary numbers Tweet

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1. Complex/imaginary numbers
For the attached question, once I see the answer it looks quite simple. But how do I know whether to use sin or cos and which number to put on top? (I'm in a bit of a hurry to go through all of the understanding properly I'm afraid) Thanks for any help! Pos rep given
Attached Thumbnails

2. Re: Complex/imaginary numbers
whether you use cos or sine depends on which "sides of the triangle" you're using. you could use either in all questions or even tan if you feel like it
Last edited by didgeridoo12uk; 09-08-2012 at 00:11.
3. Re: Complex/imaginary numbers
It's just simple trig - SOH CAH TOA.
4. Re: Complex/imaginary numbers
I use tan instead... if you do the argument as: tanx = img/real, it gives the same answer and you don't have to worry about cos or sin.

For example the first one is:
tanx = 2/3
x = tan(-1)2/3
x = 0.588 radians
5. Re: Complex/imaginary numbers
Thanks for your replies. Yeah, the only thing is the question doesn't mention which sides of the triangle these values are....so was just wondering how I know which one to use
6. Re: Complex/imaginary numbers
(Original post by cillit bang)
Thanks for your replies. Yeah, the only thing is the question doesn't mention which sides of the triangle these values are....so was just wondering how I know which one to use
Horizontal axis is real, vertical axis is imaginary.